1. Technical Field
The present invention relates generally to semiconductor devices, and more particularly, to the design of bond pads for wirebond interconnections.
2. Related Art
Wirebonding is commonly used to form an electrical interconnection between an integrated circuit and another device, e.g., lead frame, interposer, or printed circuit board. Attaching a wirebond to an active region of a chip consists of pressing a wirebond ball, having a conductive wire attached thereto, into a bond pad.
Before being used in a final product, or sent to an end user, the chip must be tested to ensure product reliability. Such testing may be performed on the chip before or after wirebonding, depending upon the design of the bond pad. A probing device is used to measure open or closed electrical systems. During probing a probe tip contacts a surface of the bond pad to measure electrical conduction. There is often a layer of oxide, or “skin”, on the surface of the bond pad that was created during formation of the bond pad. Therefore, sufficient down force is required when contacting the bond pad surface with the probe tip to penetrate the skin. Otherwise, erroneous readings may be produced, resulting in the discarding of functioning chips. Due to the downward force of the probe tip some of the bond pad material is removed during probing. As a result, the amount of bond pad material available for wirebonding is also reduced. Also, due to the uneven topography on the surface of the bond pad, the probe tip often gouges the bond pad material, leaving a pile up of bond pad material on the surface of the bond pad, and on the probe tip itself. The pile up of bond pad material on the surface of the bond pad may result in Kirkendal Voiding, wherein voids within the interconnection are produced during wirebonding as the excess bond pad material in the pile up is consumed by the wirebond ball. The excess bond pad material on the probe tip must also be cleaned frequently causing production delays.
By minimizing the amount of bond pad material, the above-stated problems are reduced. There is, therefore, a process window for bond pad thickness that minimizes the amount of bond pad removal and pile up. For example, an optimal process window for thickness of the bond pad may be 300–800 nm, e.g., 400 nm, for tight pitch wirebond products, e.g., a pitch of less than 65 nm, pitch referring to the width of the bond pad and the spacing between bond pads.
However, after the wirebond interconnection is formed some of the interconnections must withstand at least two mechanical tests, namely, a stud pull test and a ball shear test, to qualify the interconnection formation process for further use. The resistance of the interconnection to mechanical failure depends upon the pad thickness. In particular, the interconnection typically performs better during these tests as the amount of bond pad material is increased. Therefore, the process window for bond pad thickness during wirebond mechanical testing may be 800–1500 nm, e.g., 1000 nm, for a tight pitch wirebond product.
Clearly, there is a problem in forming a bond pad having sufficient bond pad material to pass the mechanical tests performed on the wirebond interconnection, and at the same time minimize the amount of bond pad material to reduce the removal and pile up of bond pad material during probing.
Therefore, there is a need in the industry for a bond pad that overcomes the above problems